Piece of cake that you eat outside? (6)
I believe the answer is:
picnic
I believe this clue is a double definition.
'piece of cake' is the first definition.
(both can mean something easy)
'that you eat outside?' is the second definition.
(a picnic gets eaten outdoors)
(Other definitions for picnic that I've seen before include "outing" , "Outdoor snack" , "Informal meal taken in the open air" , "external courses?" , "food for consumption outside" .)